Today passive optical network (PON) systems exist where an optical line terminal (OLT), usually located at a providers premises, is connected by optical fiber to a number of optical network terminations (ONTs) usually located at a residential premises or business premises. To reduce costs these networks are often passive, hence called optical PON networks, usually meaning that the routing devices between the OLT and the ONTs do not require any power or light sources. This reduces infrastructure costs greatly and means all optical sources but for a transmitter in each optical network termination (ONT) are located at the providers premises which increases safety.
In a simple arrangement, one wavelength is used for downstream signals, i.e. signals from the OLT to ONTs, and time division multiplexing (TDM) is used to allocate a portion of the signal to each ONT. A different wavelength may be used for upstream signals, i.e. signals from ONTs to the OLT. A simple passive splitter can then be used to send the correct portion of signal to each ONT.
In a refinement it is also known to use wavelength division multiplexing (WDM) whereby optical signals at more than one wavelength are sent out by the OLT, with each ONT being allocated one wavelength (although they could be allocated more than one). Each signal can then be modulated with information to be sent to an ONT. To route the correct wavelength signal to the correct ONT, a passive distribution node is provided which taps into the optical fiber from between the OLT and the ONTs. The function of the node, which often is of the arrayed waveguide grating (AWG) type, is to separate out the wavelengths and send them to the correct ONTs.
A wavelength division multiplexing passive optical network (WDM-PON) has in comparison with a (time division multiplexing passive optical network (TDM-PON) several advantages, such as increased network capacity, dedicated bandwidth to each end-user (i.e. to each ONT), communication privacy and much lower insertion loss of an AWG compared to e.g. a power splitter in a TDM-PON, which in turn enables long reach. A WDM-PON has however a disadvantage in that each ONT much transmit signals to the OLT at a specific wavelength. Since it is usually not practical to implement a larger number of ONT-types, wavelength adaptive ONT-transmitters must be used. This is typically referred to as “colorless” ONTs. Contrary to the complexity of the multitude of so called seed-source WDM-PON architectures that has been proposed, using tunable lasers as transmitters is widely considered as the best long term solution. However, apart from their current relative high cost, the problem of automatic tuning of the transmitters must also be solved. Moreover, once such ONTs are put into service, they must receive information about what wavelength they shall use for the upstream communication.
Solutions exist where a transmitter is tuned for using a correct wavelength for an upstream optical signal. KR2007059895-A, for example, discloses a WDM-PON and a wavelength initialization method thereof that provide automatically array tunable light sources according to allocated unique wavelengths. This is achieved by installing WDM-PON master controllers and a WDM-PON slave controller in an OLT and an ONT respectively. The system and method are using a table configuration algorithm and an optimal value determination algorithm for performing an automatic wavelength initialization function.
However, the solution above has the drawback that the WDM-PON and a wavelength initialization method are identification protocol specific and can not be used in case Ethernet functionality is not present, i.e. if some other layer 2 protocol is used, or it can not be used if the ONT is a layer 1 device only, or if the Ethernet functionality is not active or suitable for the used type of management information.
There are also other proposed methods, such as one using simple brute force of a laser in the ONT for transmitting an upstream signal, where the laser merely tests every upstream wavelength until the ONT receives from the OLT information that a signal has been received and understood. This method has the disadvantage that it is quite slow and that it risks corrupting the upstream communication of other ONTs if the isolation of a used wavelength splitter is inadequate.
Accordingly, the applicant has appreciated that there is a need of improvement in terms using a correct wavelength for an upstream optical signal in a WDM-PON.